The present application relates to catalytic hydrodesulfurization of sulfur-containing oils, in particular using a catalyst containing Group VIb and Group VIII metals and having a defined pore size distribution.
The need for desulfurization of oils is well known. Briefly stated, oils are needed as fuels to provide energy but most oils contain sulfur and burning these oils results in sulfur oxide pollutants. Thus, it is highly desirable to remove sulfur from the oil prior to burning the oil. Well-known catalytic hydrodesulfurization processes remove sulfur from oils by converting the organic sulfur compounds in the oil to hydrogen sulfide and sulfur-free organic compounds. The hydrogen sulfide can be relatively easily separated from the oil as hydrogen sulfide is a highly volatile, normally gaseous substance.
Typical operating conditions for hydrodesulfurization processes include a reaction zone temperature of 600.degree. to 900.degree. F, a pressure of 200 to 2000 psig, hydrogen feed rate of 500 to 15,000 SCF per barrel of oil feed, and a catalyst such as nickel or cobalt and molybdenum or tungsten on a porous refractory support.
A problem which has been recognized in the case of hydrodesulfurization of heavy oils is that if the heavy oils contain organometallic compounds the effective catalyst activity tends to decline relatively rapidly, particularly when the metals impurity is more than about 10 to 20 ppm metals such as dissolved nickel and vanadium. These metallic impurities are said to deposit on the surface and in the pores of the hydrodesulfurization catalyst.
One approach to this problem of metals impurity deactivation of hydrodesulfurization catalysts has been to alter the pore structure of the catalyst. However, the answer as to what pore structure is best has not been easily obtained and in fact there is a conflict in the answer suggested by the prior art. Exemplary patents concerned with pore size distribution of heavy oil hydrodesulfurization catalysts include the following.
U.S. Pat. No. 3,383,301 is directed to hydrodesulfurization using a catalyst satisfying the following pore size distribution:
"not more than 15 percent of the volume of the pores having a diameter in the range of 0 to 600 Angstroms in any 20 Angstroms increment of pore diameter in the range of pores having a 0 to 240 Angstroms diameter, and also having at least about 10 percent of such pore volume in pores having a diameter less than 60 Angstroms, at least 15 percent of such pore volume in pores having a diameter greater than 60 Angstroms and less than 140 Angstroms, and at least 30 percent of such pore volume in pores having a diameter greater than 140 Angstroms and less than 240 Angstroms."
U.S. Pat. No. 3,340,180 is similar to U.S. Pat. No. 3,383,301 but not as specific regarding pore size distribution. U.S. Pat. No. 3,340,180 requires less than 5 percent of the total pore volume in pores greater than 200 Angstroms in diameter and less than 10 percent of the pore volume in pores having a diameter greater than 160 Angstroms.
Three references found by hindsight in view of the present invention include Japanese publication No. 72/44001 (published Nov. 7, 1972, application No. 40,555/68) and Belgian Pat. Nos. 769,485 and 769,486, both granted Jan. 5, 1972.
The Japanese publication claims a hydrodesulfurization catalyst comprising a predominantly alumina carrier having more than 75 percent of the total pore volume in pores of 50 to 100 Angstroms in diameter, and does not specify the amount of pore volume in large pores such as pores above 1000 Angstroms in diameter.
The Belgian Pat. No. 769,485 is directed to hydrodesulfurization of residual oils using a catalyst whose particles have a pore volume greater than 0.30 ml/g, of which pore volume less than 10 percent is present in pores having a diameter of more than 1000 Angstroms and wherein the catalyst particles have an average pore diameter, expressed in Angstroms, of 75 .times. d.sup.0.9 to 170 .times. d.sup.0.9, where d represents the average particle diameter in mm. From Table A of Belgian 769,485 it is seen that catalysts with more than 3 volume percent of their total pore volume in pores greater than 1000 Angstroms in diameter gave good results in terms of the calculated deactivation rate constant, C. Catalysts with between 3 and 5 percent of their total pore volume in pores greater than 1000 Angstroms were among the very best in terms of the calculated deactivation rate constant. Three of the catalysts in Table A are listed as having less than 3 percent of the pore volume in pores greater than 1000 Angstroms. All three of these catalysts have an average pore diameter of 78 Angstroms but one of them had an unsatisfactorily high deactivation rate constant.
Belgian Pat. No. 769,486 is largely similar to Belgian Pat. No. 769,485, but the 769,486 reference is directed to a further preferred embodiment wherein the catalyst is used in suspension rather than as a fixed bed. In Table A of Belgian Pat. No. 769,486 the last column reports the calculated reaction velocity constant desulfurization in the stationary state in a reactor in which continuous renewal of the catalyst takes place with a velocity of 0.3 kilograms of catalyst per ton of fresh feed, the reaction velocity constant being labeled k.sub.e. The highest value for the calculated k.sub.e is for a catalyst having 5.9 percent of its total pore volume in pores greater than 1000 Angstroms in diameter.
U.S. Pat. No. 3,509,044 discloses desulfurization of a residuum feed using a catalyst containing Group VIb and Group VIII metals on an alumina-low silica content refractory support and with the catalyst having a major portion of its surface area in pores ranging from 30 to 70 Angstroms in diameter.
U.S. Pat. No. 3,531,398 is similar to U.S. Pat. No. 3,509,044 but is directed to hydrodesulfurization of gas oils as opposed to residuum feedstocks. U.S. Pat. No. 3,531,398 requires the use of the catalyst which has a maximum of total surface area in pores having a diameter of 30 to 70 Angstroms and a minimum in pores having a diameter greater than 100 Angstroms. The gas oil feedstocks for the U.S. Pat. No. 3,531,398 process ordinarily have a low metals content "i.e. from 1 to 50 ppm, preferably 0.05 to 10 ppm metals."
U.S. Pat. No. 3,692,698 is also similar to U.S. Pat. No. 3,509,044 but requires a major portion of surface area in pores of 30 to 80 Angstroms in diameter and less than 4 percent of the total pore volume in pores of 200 to 2000 Angstroms, and at least 3 percent of the total pore volume in pores greater than 2000 Angstroms in diameter. Thus, U.S. Pat. No. 3,692,698 teaches that hydrodesulfurization catalysts should have a substantial amount of macropores, namely at least 3 volume percent of their pore volume from pores greater than 2000 Angstroms in diameter.